This invention relates to impregnants for electrical capacitors having metallized paper electrodes, and more particularly to round roll capacitors which utilize a pair of doubly metallized paper strips as electrodes and an intermediate synthetic resin strip as the dielectric.
An electrical capacitor in a simple form comprises a pair of spaced electrodes, usually aluminum foil, with a synthetic resin dielectric film therebetween. If the aluminum foil is replaced by a suitably deposited very thin aluminum coating on the film the capacitor is referred to as a metallized capacitor. A metallized capacitor is most desirable in some applications because of its inherent self-clearing characteristics, i.e., when an electrical short occurs between electrodes, the arc burns away this thin coating in an enlarging area until the arc extinguishes. A typical capacitor of this kind is found in U.S. Pat. No. 3,987,348.
A recent and improved metallized capacitor uses as the electrodes a strip of paper having both sides covered by a thin metal coating and separated by a synthetic resin strip. In this arrangement the electrodes and the resin strips are rolled in convolute form, inserted in a suitable casing impregnated with a dielectric fluid. A typical tightly wound, round, metallized capacitor of this kind utilizing a liquid impregnant is found in U.S. Pat. No. 3,555,642.
A significant problem with the latter described capacitor relates to the need to improve certain characteristics such as corona start voltage, and capacitance loss with time. These problems may be influenced by the use of different fluids. However, the fluid/dielectric swelling ratio of the synthetic resin film in this kind of capacitor is most critical and seriously limits the changes brought about by the use of different fluids with different dielectric constants, viscosities, and other characteristics, or mollifies them.
One of the predominant capacitors of the type described utilizes a low dielectric constant fluid as the optimum fluid for this kind of capacitor. U.S. Pat. No. 3,555,642, Hagedorn, discloses an example of the kind of capacitor referred to and as one criterion indicates the relationship between the swelling of the film dielectric and the fluid penetration to be extremely important and that swelling of the film should be carefully correlated to the impregnating fluid and the space factor in the roll.
In contradistinction to the above combination, it has been discovered that such a capacitor can be dramatically improved by special blends of lower and higher dielectric constant fluids which in fact are blends of higher and lower swelling fluids. Also these blends are combinations of specified esters and selected hydrocarbons.